1. Field of the Invention
The present invention relates to an information recording medium, and more particularly to a phase-change type of information recording medium wherein a phase change, resulting from the application of a laser beam, is produced in a material which forms a recording layer to provide an information recording medium on which recording or writing (hereinafter referred to as writing or write), reproduction, and overwriting of information is possible, and which is suitable for application in optical memory related equipment.
2. Discussion of Background
One commonly known type of optical memory medium whereon writing, reproduction and erasing of data is possible by the application of electromagnetic waves, in particular, a laser beam, is the phase-change type of recording medium which utilizes the transition from a crystalline phase to a non-crystal or amorphous phase or from one crystalline phase to another crystalline phase. In particular, with respect to this type of recording medium, research and development has recently become very active because overwriting using a single beam, which is difficult in the case of a magneto-optical memory, is possible., and the optical system on the drive side for this recording medium is simpler in mechanism than that of the optical system for a magneto-optical memory.
As representative examples of a material for the phase-change type recording medium, chalcogen type alloy materials such as Ge-Te, Ge-Te-S, Ge-Se-S, Ge-Se-Sb, Ge-As-Se, In-Te, Se-Te, Se-As and the like, as disclosed in U.S. Pat. No. 3,530,441, can be given. Furthermore, it has been proposed to add to the Ge-Te alloys materials such as Au as disclosed in Japanese Laid-Open Patent 61-21962, Sn and Au in Japanese Laid- Open Patent 61-270190, and Pd in Japanese Laid-Open Patent No.62-19490 with the object of improving the stability and speed of crystallization. A material with a specified composition ratio for Ge-Te-Se as disclosed in Japanese Laid-Open Patent 62-73438 has also been suggested for improving the write/erase repeatability characteristics. However, none of these methods completely satisfy the many characteristics required in a phase-change type optical memory medium in which overwriting is possible. In particular, the most important problems to be solved are the improvement of write and erase sensitivity, prevention of lowering of erase ratio resulting from failure of erase prior to overwriting, and improvement of the life of both the recorded and unrecorded portions in the recording layer.
In the phase-change type of recording method, the transition from a crystalline phase to an amorphous phase or from one crystalline phase to another crystalline phase is utilized, and the energy from the incident electromagnetic waves is converted into heat energy in the recording layer portion so that a phase transition occurs between the recorded section and the non-recorded section of the recording material. In order to reduce the time required for writing and erasure as far as possible, an amorphous phase, which is a metastable phase, is generally used, for example, in the recorded portion, and a crystalline phase, which is a stable phase, is generally used in the unrecorded portion. To form the amorphous phase which is a metastable phase, the temperature of the material must be raised above the melting point (Tm) to break the bonds between the molecules. Furthermore, rapid cooling is necessary to freeze the disordered or irregular state in the amorphous phase. To form the crystalline phase which is a stable phase, the temperature of the material must be raised above the crystallization transition point (Tc) to stimulate the bonds between the molecules. Also, slow cooling conditions are necessary to form the crystalline phase. From this type of theoretical reasoning, in a phase-change type of recording material a breakdown in the characteristics of the medium and a shortening of the life span are unavoidable from the thermal history.
The characteristics such as writing and erasure sensitivities and life span are strongly influenced by the magnitude of the barriers to energy transition between the amorphous and crystalline phases, specifically, the melting point (Tm) and the crystallization transition point (Tc). When these energy barriers are small, the writing and erasure sensitivities are good, but the life of the writing portion is short. Conversely, when these energy barriers are large, the writing and erasure sensitivities deteriorate but the life span is long. Accordingly, there are many cases in which a material is used in which the melting point is usually about 600.degree. C. and the crystallization transition point about 200.degree. C. so that these conditions are optimized. In order to provide an improvement in sensitivity, there are also cases in which a material with a large heat absorption ratio is added to the recording layer, or in which a heat absorbing layer or the like is provided in the recording medium. However, elevating the temperature of the recording material close to the melting point causes a degradation of the writing and erasure characteristics from the thermal history. Furthermore, when consideration is given to the oscillation output of a semiconductor laser, if a recording medium which needs high write power is employed, the cost of the recording apparatus for such a recording medium will also be high.
High speed and high writing density are also expected in an optical recording medium, and writing and erasure under these conditions require even higher power which causes lowering of the writing and erasure sensitivities, C/N ratio, and erase ratio.
In Japanese Laid-Open Patent 63-251290, an optical recording medium is disclosed in which a recording layer, when in a crystalline state, is of a single phase comprising a multi-component (tertiary or more) compound. Here, in practice, a compound with a tertiary or greater stoichiometric composition, for example, In.sub.3 SbTe.sub.2, is used as the multi-component compound with a 90 atomic % or greater in the recording layer. By using this type of recording layer, high speed writing and erasure are possible. However, the laser power required for writing and erasure has not yet been sufficiently reduced. There are other drawbacks inasmuch as the erase ratio is also low, and the repeatability and long term reliability are not satisfactory. The development of a suitable recording material for providing stable, highly sensitive writing and erasure is still awaited.